NEW YORK (AP) - Allies of Vermont Sen. Bernie Sanders are rallying behind the embattled presidential prospect even as they reluctantly begin to ponder the painful possibility of a 2020 campaign without him.

The 77-year-old self-described democratic socialist is the most prominent contender to face a serious setback in the evolving White House field. He's been forced to confront reports detailing allegations of sexual harassment of women by male staffers when he sought the Democratic nomination for president in 2016.

No one has alleged that Sanders had direct knowledge of the incidents.

Sanders' loyalists expect him to launch a second campaign in the coming weeks, and his network of die-hard supporters is hosting hundreds of events across the nation this weekend encouraging him to run.

But the allegations put Sanders in an unenviable position in the early days of a contest playing out in the #MeToo era. While his competitors are visiting early-voting states and scoping out potential campaign headquarters, Sanders spent Thursday apologizing for the behavior of a handful of 2016 campaign workers and looking for new staffers should he run in 2020.

Some allies have had their confidence shaken in Sanders' future, although he's reshaped Democratic politics and almost single-handedly and brought liberal priorities such as "Medicare for all" and free college education into the party's mainstream.

"If he doesn't run, there's a massive void in this country," said RoseAnn DeMoro, an activist and former executive director of the National Nurses United union, who reaffirmed her support for Sanders. "The passion in that base goes away. That base evaporates. It doesn't go to someone else. There would be a void so deep it would go to (President Donald) Trump, I suspect."

Politico reported Wednesday that in July 2016, a former senior Sanders adviser forcibly kissed a young female staffer after making sexually explicit comments. Sanders' team said the adviser, who denies the allegation, would not be involved in any second campaign. Former campaign manager Jeff Weaver, who was made aware of some incidents after the 2016 campaign ended, would not serve in that capacity again.

"Obviously, it's impacted all of us quite a bit. It's very upsetting," said Heather Gautney, executive director of Our Revolution, the political arm of Sanders' network.

Despite her concern, Gautney warned Democrats that a 2020 contest without Sanders would undermine plans to shake up health care, education, housing as well as other liberal priorities.

"Bernie is holding the flank on the left. If he doesn't run for president, then the whole horizon shifts, and universal health care maybe gets taken off the table," Gautney said. "In my view, he is an absolutely necessary part of our political system."

Sanders would be a force in 2020. Having nearly beaten Hillary Clinton in the 2016 contest, he boasts an engaged nationwide network and impressive grassroots fundraising ability. He almost certainly would draw some of the same voters being courted by likely 2020 contenders such as Massachusetts Sen. Elizabeth Warren, New Jersey Sen. Cory Booker and former Texas Rep. Beto O'Rourke.

Former Sanders' staffer Giulianna Di Lauro Velez, who alleged she was harassed during the 2016 campaign, wrote Thursday in The Intercept that sexual harassment is prevalent in many political campaigns. But new allegations about Sanders' 2016 campaign indicate "the problem was likely deeper than most knew."

She called on Sanders to "take the rare step of setting up an independent investigation into the 2016 allegations."

Sanders' Democratic colleagues declined to address his situation, but some spoke to the broader issue of sexual harassment on campaigns.

"I think that no workplace is immune from those issues, and we should take it seriously," California Sen. Kamala Harris said.

Sen. Mazie Hirono of Hawaii, went further: "Campaigns, offices, everyone should have rules of the road and policies on how we will deal with concerns about sex harassment and sexual assault and retaliation. That's a good idea, across the board, but definitely for campaigns because that's another kind of situation where you have power situations among volunteers."

Earlier in the day, Sanders apologized, as he did last week, for harm done under his watch and offered a direct message to women affected.

"I thank them from the bottom of my heart for speaking out. What they experienced was absolutely unacceptable and certainly not what a progressive campaign - or any campaign - should be about," Sanders said.

Sanders' critics in the Democratic Party have seized on the new revelations as reason for him to abandon any 2020 plans.

"These allegations inform us that Bernie is really not concerned about the well-being of women. And therefore, he would not represent us well as the president," said Toni Van Pelt, president of the National Organization for Women. "I really think Bernie needs to sit down."

Sanders' vast political network does not agree.

Katherine Brezler, co-founder of People for Bernie Sanders, said the allegations of sexual harassment had absolutely no impact on her support. The New York activist said that sexual harassment was present in virtually every one of the 100 or so campaigns she's worked on.

"I've met those people and they're not Bernie Sanders," Brezler said. "We are not going anywhere."